1. Field of the Invention
This invention relates generally to protective devices that may include Arc Fault Protection or Ground Fault Protection or both, and more particularly to a protective device comprising fail safe features.
2. Description of the Prior Art
A percentage of fires each year is caused by electrical branch circuit line arcing which is of a duration, and at a level, that does not activate the thermal or magnetic trip elements in conventional circuit breakers in time to prevent a fire. A high percentage of electrocutions each year is caused by a current flow through the body to ground, the level of which is too low, to activate the thermal or magnetic trip elements in conventional circuit breakers in time to prevent electrocution.
Arc detection is an enhancement to thermal magnetic overload detection typically used in circuit breakers, which otherwise may not detect and respond to arc faults. A number of devices for detecting arc faults and methods of detection have been used in the past. These include the use of E and B field arc sensors, detecting the amplitude of the rate of change of current signals when an arc fault occurs, the use of non-overlapping band pass filters to detect white noise that is characteristic of arcs, and detecting the disappearance of signals indicating the presence of arcs near zero current crossings. While some of these techniques are more or less effective, they require relatively sophisticated arc sensors and circuits and heretofore, most of these arc detection circuits have been incorporated in circuit breakers.
A number of devices and methods for detecting ground faults have been used in the past. Typically ground faults are detected using B field sensors for sensing a difference between line neutral current together with integrators or low pass filters and are more or less effective. Heretofore, ground fault detection circuits have been incorporated in circuit breakers or receptacles.
There is a need for simple economical arc fault detectors that can be included in wiring devices such as receptacles, plugs, or in-line devices, and that offer the same protection as an arc fault detector incorporated in a circuit breaker, but at lower cost. There is a need for an arc fault circuit interrupter (AFCI) in wiring devices that can be provided at a reduced cost compared with arc fault circuit detecting circuit breakers that is comparable to the difference in cost between ground fault interrupting receptacles and ground fault interrupting circuit breakers. There is the need for a sensor and associated circuitry that are miniaturized.
There is need for simple economical ground fault detectors that are either independent or a part of an arc fault device and that can be included in wiring devices, but also at lower cost.
There is a need for improved device reliability and for reporting to the user of the protective device if there is a device malfunction, and for reporting within a short time interval after the occurrence of the malfunction.
It is an object of this invention to provide an arc fault circuit interrupter, also known as AFCI, that employs an electrical circuit that is simple enough, inexpensive enough and small enough to be included in wiring devices.
It is another object of this invention to provide an arc fault circuit interrupter that is sensitive to relatively low amplitude series arc faults of at least 5 amps of arc current, typically in series with the load and commonly referred to as Type A faults.
It is another object of this invention to provide an arc fault circuit interrupter that detects parallel or line to line arcs, producing currents of 75 amps or more, commonly referred to as Type B arc faults.
It is another object of this invention to provide a ground fault circuit interrupter that detects power line leakages to ground, typically at 60 Hertz, whose current is 5 milliamps or more.
It is a principal object of this invention to provide all of the above identified features in an arc fault detector that monitors its own operation, and deactivates itself in the event of a malfunction.
This invention discloses a Protective Device having an automatic, self-test feature that determines if the device is operational. The self-test feature is accomplished with miniature, low cost electronic components. This allows for the protective device with the additional self-test feature to be constructed to fit into a wiring device sized package and which may also permit a dual-purpose arc and ground fault detection circuit in the same package.
Briefly stated, and in accordance with a presently preferred embodiment of the invention, an arc fault or ground fault detector for detecting disturbance on the electric power lines includes at least one sense transformer, at least one detector for determining whether the sensed signal exceeds a threshold, a processor for analyzing the timing characteristics of the detected signal, a trip mechanism that is enabled by the processor if pre-established timing characteristics are discerned from the sensed and detected signal, and interrupting contacts in at least one of the electric power lines for interrupting power to the protected terminals of the device in response to the enabled trip mechanism. The processor also receives signal from a clock, and the clock also generates a recurring signal to test one or more components of the device. If one or more components is malfunctioning, the processor determines that the test failed and the processor delivers a signal to an indicator to report the malfunction to the user. In another embodiment, a computer operating properly (COP) timer is interposed between the processor and the trip mechanism. If all components including the processor are operational, the processor outputs a recurring signal to the COP, timer, then the COP timer does not activate the trip mechanism, and the interrupting contact remain closed. If there is an interruption of the recurring signal from the processor, then the COP timer enables the trip mechanism, and the interrupting contacts are opened. In this way, the device components are self-tested. Alternatively, the processor can produce a second recurring signal derived from the clock, the second recurring signal not influenced by the test result previously described. The second recurring signal occurs at a higher rate than the first recurring signal in order to reduce time constants within the COP timer, in order to facilitate construction of the COP timer. The first recurring signal is operated at a comparatively lower rate so that the time required to self test the device does not interfere with the ability of the device to detect an occurrence of a true fault condition. Taken together, the second recurring signal connected to the COP timer checks the operation of the processor, and the first recurring signal tests the components of the device as previously described.
The protective device contains a clock, which generates a recurring clock signal. The clock signal initiates a plurality of test signals upon each occurrence. The test signals serve to evaluate the various electrical and mechanical elements of the protective device, the operational status of the protective device being displayed using at least one indicator. In another embodiment of the invention, the protective device receives the recurring clock signal from a remote master controller and may report the operational status of the protective device back to the master controller. In yet another embodiment, if the plurality of test signals each yield an acceptable test result, an enabling signal is generated. The enabling signal recurs with each acceptable test result to maintain closure of the interrupting contacts, consequently the protective device delivering power to the protected terminals of the device. If there is a lapse of recurring enabling pules for a pre-established period, the interrupting contacts open, and the protective device is disconnected from the source of power. Alternatively, the enabling signals can recur at a second recurring rate or the enabling signals can continue regardless of the test result, the disappearance of enabling signals representing different modes of device failure.